1. Field of the Invention
The invention is directed to control of prefire timing, ink ejection power, and pulse width modulation for a print head used with an ink jet printing apparatus. More particularly, the invention relates to such control based on ink ejection and prefire timing and changes in a number of driven print head nozzles.
2. Description of the Related Art
Ink jet printers print images by ejecting ink through nozzles of print heads onto recording media. In order to form different images, different nozzles are driven so as to eject ink while the print head or print heads are scanned across a recording medium. Depending on the image that is recorded, some nozzles may be unused for multiple scans. Residual ink in the unused nozzles tends to dry or coagulate, clogging the nozzles. Then, when then ink jet printer attempts to use the clogged nozzles, the nozzles might not eject ink properly, resulting in poor image quality.
One conventional technique for maintaining unclogged nozzles is to periodically prefire the nozzles. Prefiring the nozzles involves moving the print head or print heads to a home position in the printer and then ejecting ink from the nozzles, thereby clearing the nozzles of drying or coagulating ink. Convention systems perform this prefire operation at a regular interval.
Moving the print head or print heads to the home position and ejecting ink from the nozzles takes considerable time. As a result, performing the prefire operation at a short time interval unacceptably slows the image forming operation. However, performing the prefire operation at a long interval may not be sufficient to maintain unclogged nozzles and high image quality. In particular, unexpected interactions between high-end host processors and printing devices that use long intervals have been found to result in significant image degradation, as explained below in section 9.0 with reference to FIGS. 9-2a through 9-2d. Accordingly, there exists a need for control of prefiring and nozzle operation that provides acceptable image formation speed while maintaining unclogged print head nozzles and high image quality.
The present invention addresses the foregoing need by controlling the prefiring operation to be performed when the data to be printed requires driving nozzles that have not been driven for a first time interval since a previous prefiring operation, when none of the nozzles have be driven for a second time interval, or when a third time interval has elapsed since the previous prefiring operation. The first and second time intervals are shorter than the third time interval.
By virtue of the foregoing, prefiring is performed in some cases after the shorter time intervals, while in other cases prefiring is delayed until the longer time interval has elapsed. This combination of prefiring control tends to reduce the overall number of prefirings, thereby increasing the speed of printing operations, while still performing sufficient prefirings to maintain unclogged print head nozzles.
In addition, according to the invention, a power of ejecting ink from the nozzles is preferably enhanced after the first time interval has elapsed since the previous prefiring operation. Use of enhanced power for one nozzle also tends to reduce clogging in adjacent nozzles in a print head. Thus, even though a prefiring operation has not been performed for the first time interval, clogs tend to be prevented by the enhanced ejection power.
Accordingly, the invention in one aspect is an ink jet printing apparatus which performs printing by using a print head. The print head has at least a predetermined number of nozzles to eject ink. The apparatus includes first prefiring means for causing the print head to perform a prefiring operation to eject ink from nozzles of the print head for maintaining printing quality after a first time interval during a printing operation. The apparatus also includes driving means for driving nozzles of the print head based on data to be printed, and second prefiring means for causing the print head to perform the prefiring operation in a case where a number of the nozzles to be driven is changed.
By performing a prefiring operation in a case where a number of nozzles to be driven is changed, ejection quality for the newly-driven nozzles tends to be enhanced. Thus, a longer time interval can be used between prefiring operationes.
Preferably, the apparatus further includes shifting means for causing the driving means to drive the nozzles of the print head based on the first time interval during the printing operation so as to enhance a power of ejecting ink and continue the printing operation after the first time interval without performing the prefiring operation. The shifting means also preferably delays the prefiring operation to a second time interval longer than the first time interval, and after the second time interval, the first prefiring means causes the print head to perform the prefiring operation. In a case where the number of nozzles to be driven is changed within a time duration after the first time interval and during the second time interval, the second prefiring means causes the print head to perform the prefiring operation.
By enhancing a power of ejecting ink without performing the prefiring operation after the first time interval, and by delaying the prefiring operation until after the longer second time interval, the number of prefiring operations tends to be reduced while maintaining ink ejection quality.
The ink jet printer also preferably has at least a first mode for high printing speed and a second mode for high printing quality. The shifting means is activated in the first mode. Thus, less prefirings tend to occur in the high speed mode, improving print speed. In contrast, more prefirings tend to occur in the high quality mode, thereby tending to enhance image quality.
The invention also preferably includes detecting means for detecting a status of the print head. The driving means drives the nozzles of the print head in a first driving manner based on the data to be printed and the status detected by the detecting means, and the shifting means causes the driving means to drive the print head in a second driving manner different from the first driving manner. The driving means preferably drives the print head in the first driving manner with pulse signals determined based on the data and the detected status, and the shifting means causes the driving means to drive the print head in the second driving manner with pulse signals having a predetermined pulse width. The predetermined pulse width preferably is a maximum pulse width, thereby enhancing ink ejection. Thus, the shifting means tends to enhance ink ejection after the first time interval, thereby compensating for the driving means not performing the prefiring operation.
In another aspect, the invention is an ink jet printing apparatus which performs printing by using a print head. The apparatus includes prefiring means for causing the print head to perform a prefiring operation to eject ink for maintaining printing quality after a first time interval during a printing operation. The apparatus also includes driving means for driving the print head based on data to be printed, and shifting means for causing the driving means to drive the print head based on the first time interval during the printing operation so as to enhance a power of ejecting ink and continue the printing operation after the first time interval without performing the prefiring operation. Preferably, the shifting means delays the prefiring operation to a second time interval longer than the first time interval, and after the second time interval, the prefiring means causes the print head to perform the prefiring operation.
In the foregoing arrangement, the shifting means advantageously tends to reduce a number of prefirings while causing the driving means to maintain unclogged nozzles.
In yet another aspect, the invention is a method in an ink jet printing apparatus which performs printing by using a print head which has at least a predetermined number of nozzles to eject ink. According to the method, a prefiring operation is performed to eject ink from nozzles of the print head for maintaining printing quality after a first time interval during a printing operation. Nozzles of the print head are driven based on data to be printed, the prefiring operation is performed in a case where a number of the nozzles to be driven is changed. Preferably, the nozzles of the print head are driven based on the first time interval during the printing operation so as to enhance a power of ejecting ink and continue the printing operation after the first time interval without performing the prefiring operation.
In yet another aspect, the invention is a method in an ink jet printing apparatus which performs printing by using a print head. The method includes the steps of prefiring the print head to eject ink for maintaining printing quality after a first time interval during a printing operation, driving the print head based on data to be printed, and driving the print head based on the first time interval during the printing operation so as to enhance a power of ejecting ink and continue the printing operation after the first time interval without performing the prefiring operation. The method also preferably includes the step of delaying the prefiring operation to a second time interval longer than the first time interval, and after the second time interval, performing the prefiring operation.
The foregoing methods both also tend to reduce a number of prefirings while maintaining unclogged nozzles so as to ensure good image quality.
This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiment thereof in connection with the attached drawings.